The present invention relates generally to the field of mechanical connections, and more particularly to the connection of a tube to a machine, and specifically to a method and apparatus for avoiding fretting failure in a tube connected to a machine.
It is known that adjoining mechanical components subjected to alternating stresses between the components are subject to fretting of the contacting surfaces of the components. Of particular interest to the assignee of the present invention is the connection of a fuel tube to an engine, such as the engine of a locomotive. It is known to attach a fuel tube to an engine by using a compression fitting, wherein a tapered collet is compressed against a fuel tube by threading a nut onto the engine or onto an attachment thereto. Vibration produced by the engine during operation imparts an alternating load on the tubing in the region of contact with the collet of the compression fitting. Such alternating mechanical loads can produce alternating micro-movements between the tubing and the collet, which in turn may produce fretting of the tube and/or collet surfaces. Cracks produced in the tube wall as a result of such fretting may result in failure of the tubing if the loading on the tubing is sufficiently high.
It is known in the prior art to reduce or eliminate fretting by forming grooves in one or both abutting surfaces subject to alternating loads. For example, U.S. Pat. 3,976,342 issued on Aug. 24, 1976, to Leyendecker, et al., describes how closely-spaced cavities are used to reduce fretting wear between relatively moving parts. Similarly, U.S. Pat. 4,468,148 issued on Aug. 28, 1984, to Seymour, describes the use of slots on abutting surfaces on a vibrating part. However, the applicant has found that the use of such cavities or slots in the application of a tube in a compression fitting is unacceptable since the resulting stress concentration created by a cavity or slot can result in the failure of the tube independent of any fretting damage. U.S. Pat. 5,080,406 issued on Jan. 14, 1992, to Hyatt, et al., describes a fitting for a tube that has a balanced configuration in order to minimize fretting. The balanced configuration eliminates the axial movement between the tube and the fitting, and is achieved by taking into consideration the loadings, moduli of elasticity and restraining conditions. Designing for such application-specific variables makes the apparatus of Hyatt a costly and non-standard component.
There is a particular need for an improved method and apparatus for avoiding fretting failure in a tube connected to a machine. A connection to a machine is described herein comprising: a fitting adapted to be connected to a machine and adapted to receive an end of a tube for attachment thereto, the fitting comprising a collet and a nut for compressing the collet onto the outside surface of a tube; a tube having an end disposed within the fitting and having an area in contact with the collet; a groove formed on the outside surface of the tube proximate the collet, and a region of residual compressive stress in the tube proximate a bottom surface of the groove. In one embodiment described herein, the groove is formed in a thread pattern extending from a point on the tube outside the area of contact with the collet to a point within the area of contact with the collet. A method for connecting a tube to a machine is also described herein, the method comprising: forming a groove on the outside surface of a tube in a manner providing of region of residual compressive stress in the tube proximate the bottom of the groove; and installing the end of the tube into a compression fitting attached to the machine so that the groove is proximate an end of the fitting opposed the machine. In one embodiment, the method includes the step of rolling the groove into the outside surface of the tube in a thread pattern. In a further embodiment of the invention, the method includes the step of forming a second groove on the outside surface of the tube proximate the first groove, and further, positioning the tube so that the first groove is located in an area of the tube in contact with the fitting and the second groove is located outside the area of the tube in contact with the fitting.